1. Technical Field
This disclosure relates to an aberration correction (optical) element that corrects aberration to obtain compatibility for reading and writing data on multiple kinds of optical recording media having different recording density.
2. Discussion of the Background
Optical recording media such as CDs having a storage of 0.65 GB and DVDs having a storage of 4.7 GB have been diffusing as a device to store or save audio visual data or data on computer. In recent years, a further increase in the recording density and storage (data capacity) has been strongly demanded.
Such a demand is satisfied by, for example, increasing the number of numeric apertures (NA) of an objective lens in an optical pickup that reads and writes data on an optical recording medium, or reducing the size of a beam spot focused and formed on an optical recording medium by an objective lens by shortening the wavelength of the light emitted from a light source.
The numeric aperture of the objective lens and the wavelength of the light source for compact discs (CDs) are 0.50 and 780 nm, respectively and those for DVDs having a higher recording density than CDs are 0.65 and 660 nm.
As described above, an optical medium having a higher recording density and a larger data capacity has been demanded these days, meaning that the numeric aperture of an objective lens greater than 0.65 and a light source that emits light having a wavelength shorter than 660 nm are demanded.
The specification of Blu-ray disc (hereinafter referred to as BD) was proposed as an optical recording medium and an optical data processing device for a larger data capacity.
That is an optical recording medium that secures a data capacity corresponding to 22 GB by using an objective lens having a numeric aperture of 0.85 and a light source that emits light having a wavelength in the blue color range.
In addition, an optical pickup that reads and writes data on a high capacity optical recording medium such as a BD is also demanded to secure reading and writing data on existing optical recording media such as CDs and DVDs which have been supplied into the market in large quantity.
Selecting a light source that emits light having a suitable wavelength according to the kind of optical recording medium to be read or written and conducting suitable optical treatment to the selected beams of light to correct spherical aberration caused by the thickness difference among the substrates of optical recording media is preferable.
A technology of using a single optical pickup describes a method and/or a device of using two objective lenses to read and write data on different kinds of optical recording media. Another technology describes a method and/or a device of using a single objective lens for different kinds of optical recording media to reduce the number of parts and size.
Yet another technology describes a method and/or a device of using a diffraction lens to compensate the deterioration of focus light spot formation power to the data recording plane of an optical recording medium caused by a temperature change inside the optical pickup.
However, the first mentioned technology uses two objective lenses, which leads to an increase of the number of parts. This is not preferable in terms of size reduction and cost reduction. Furthermore, the objective lenses are moved and switched according to the kind of the optical recording media, which causes problems such that the mechanism of the actuator is complicated, and the access time on data takes a longer time.
In the second mentioned technology, the objective lens is made of glass. That is, the weight of the objective lens is heavy, which results in heavy burden on the actuator. Furthermore, the objective lens made of glass is difficult to manufacture by injection molding, and is relatively expensive in comparison with an objective lens made of plastic.
When a light and inexpensive plastic objective lens manufactured by injection molding is used, the form and the refraction factor of plastic lens and the oscillation wavelength of the beam of light emitted from the light source vary depending on the temperature change inside the optical pickup. This change causes aberration of the focus spot, which may prevent precise reading and writing.
The third mentioned method and/or a device uses a single objective lens having a phase structure for temperature compensation on the curved surface of the objective lens. However, manufacturing a phase structure on the non-curved surface of an objective lens satisfying an NA of 0.85 for blue wavelength is difficult. In addition, the optical utilization efficiency decreases due to the diffraction structure for use in the temperature compensation mechanism.
An objective lens having no temperature compensation structure is also described but a temperature compensation plane is provided in addition to two aberration correction planes. Thus, the number of parts increases, which is contrary to the idea of the size reduction and cost reduction.